The fuel cell is capable of producing electricity without a substantial need for fossil fuel that poses concerns about resource depletion, without noise, and at a high energy recovery rate as compared with other energy-based power generating systems. Great efforts have been made to exploit the fuel cell as a power generating plant of relatively compact size in buildings and factories, with some cells having been commercially implemented. In particular, polymer electrolyte fuel cells (PEFC) featuring compactness can operate at lower temperature than fuel cells of other types. The PEFC then draws attention not only as a device for household co-generation, but also as the replacement power source for internal combustion engines on vehicles because of the minimized corrosion concern regarding the materials of which cell components are made and their ability to discharge relatively high current flow despite low temperature operation. The PEFC is constructed of electrolyte membranes, separators and other components. The separator is generally a plate which is provided with a plurality of parallel channels on one surface or both surfaces. The separator plays the role of conducting the electricity produced at the gas diffusion electrode within the fuel cell to the exterior, discharging water produced within the channels in the course of electricity generation, and securing the channels as a flow path for reaction gas input to the fuel cell. Such a fuel cell separator is required to be more compact in size. Since a plurality of separators are used in stack, there is a demand for a separator seal material having durability and long term service. Recently, it becomes a practice to apply a seal material to electrolyte membranes themselves to establish a better seal. In this case, silicone rubber must be fully adherent to not only separator substrates, but also electrolyte membranes. Additionally, its adhesion must last long because it must maintain acid resistance, water resistance and heat resistance.
While several methods are employed in the art for bonding silicone rubber to separators, one common method is by applying a primer to a separator substrate during its molding and bonding a silicone rubber composition thereto while molding it over the substrate. The method, however, includes cumbersome steps as demonstrated by unevenness of primer coating and drying variations, which cause some troubles. One approach taken to avoid such inconvenience is by using a silicone rubber composition having a tackifier previously admixed therein. This eliminates the step of primer coating and reduces the number of steps. For example, JP-A 8-53661 corresponding to U.S. Pat. No. 5,536,803 describes that an addition curable silicone rubber composition is made adherent to organic resins by adding thereto a tackifier having at least one silicon-bonded hydrogen atom and a phenyl structure in a molecule. Also JP-A 2002-201454 corresponding to U.S. Pat. No. 6,780,518 describes that a silicone rubber composition exerts selective adhesion to organic resins by adding thereto an organic compound or silicone having an epoxy radical and an aromatic ring. However, the compositions of these patents are not intended for use in fuel cells, and in fact, they are not fully adherent to electrolyte membranes, typically fluorochemical membranes. In the fuel cell separator sealing application, JP-A 2006-176688 discloses an addition curable silicone rubber composition which is made self-adhesive by adding a tackifier thereto. Although this composition is adherent to electrolyte membranes at the initial, the once established adhesion undesirably fails within a short time in an environmental test such as a hot water test.